This invention relates to a circuit operation mode and control thereof suitable for igniting a high intensity (high pressure) discharge lamp without failure and at low cost and with low stresses on the circuit. The lamp is ignited by high frequency voltage and operated by a low frequency waveform.
In conventional electronic ballasts, specifically pulse started metal halide discharge lamp ballasts, an extra ignitor circuit is required. The ignition pulse amplitude specified by the lamp manufacturer is very high though it varies among manufacturers. Also, the width of the pulse has a minimum required value, typically 1.5 microseconds to 2.5 microseconds. To meet the starting pulse requirement, the ignitor may be complicated and expensive.
For example, FIGS. 1 and 5 illustrate circuit topographies found in, for example, U.S. Pat. No. 5,932,976, in which a high frequency resonant ignition technique is described. As shown in FIGS. 2 and 3 (or FIGS. 6 and 7), the lamp is operated using a high frequency starting operation and a low frequency steady-state operation. When the circuit is operated in the high frequency starting mode, a 50% duty-cycle high frequency square waveform is applied to the resonant circuit, formed by the inductor and capacitor, producing a high voltage to ignite the lamp. After detection of a lamp ignition, the switching mode changes to a low frequency steady-state operation. When the operating mode is changed from the starting operation to the steady-state operation, there is a delay time (as shown in FIGS. 4 and 8) which is caused by the filtering circuit of the detecting circuit. This delay causes instability of the discharge during transition from the glow to the arc, and the lamp may be easily extinguished. This phenomenon frequently happens when the lamp is not cooled down completely. In the illustrated circuit topology, the power control stage and the inverter stage are combined in a half-bridge/full-bridge topology. Since the power control (buck) stage is combined with the output inverter and the power circuit is working in a discontinuous current mode in a steady-state operation, the output inductor and the capacitor across the lamp must provide sufficient filtering to keep the high frequency component of the lamp current to a minimum in order to prevent an acoustic resonance. Consequently, the value of the capacitor must be large, in the order of, but not limited to, for example, {fraction (1/10)} micro farads. Because of the large capacitance value and relatively small inductor value, a very large circulating current flows in the circuit during the high frequency starting operation.
In U.S. Pat. No. 5,182,702, a control scheme of unbalanced duty-cycle operation for a dimming control is described. The unbalanced duty-cycle operation control allows the ballast output current to stably be controlled at a fixed switching frequency.
The present invention overcomes the disadvantages of the above-mentioned technique and keeps the advantage of high frequency starting. The objective of the invention is to overcome the problems of the lamp extinguishing due to the delay time and the high circulating current in the prior art. Another objective of the invention is to simplify the circuit for igniting HID lamps and reduce the cost of the HID ballast.
The circuit of the present invention utilizes a half bridge (shown in FIG. 9) or a full bridge (shown in FIG. 12) inverter circuit to provide the HID lamp with low frequency current. The inverter also controls the lamp power and current. In the new igniting control scheme, at starting mode, the duty-cycle of the switches is modulated by the required output low frequency square waveform or a fixed amplitude DC signal and the voltage on the lamp is composed of the high frequency, high voltage plus the low frequency square waveform voltage or DC voltage as shown in FIGS. 11, 14, and 16. It is the high frequency, high voltage that breaks down the lamp and it is the low frequency square waveform voltage or DC voltage that pushes the lamp from the glow to the arc as the lamp is broken down. In this way, the lamp current can be set up in no delay time and then the lamp current is controlled by the current loop. Therefore the lamp can be ignited without failure.
In the inverter, during the steady-state, the current in the output inductor, which is a buck inductor, is continuous. There are separate current and voltage loops in the control circuit to control the lamp power and the lamp current. The current in the output inductor can be considered as a controllable current source. Due to the continuous current of the output inductor, the current ripple is low. Because of the low ripple on the output current, it isn""t necessary to use a second order filter formed by the output inductor and the capacitor parallel with the lamp to filter out the high frequency ripple current through the lamp to avoid acoustic resonance. In this case, the value of the capacitor parallel with the lamp will be determined by the lamp ignition. Due to the low value of the capacitor, it is possible to make the resonant tank circulating current as low as possible and obtain a voltage magnitude high enough to ignite the lamp during the lamp igniting mode. Because of the low value of the capacitor, the output impedance of the resonant tank is too high to push the lamp from the glow to the arc and the lamp would be extinguished easily. However, a good lamp start condition is achieved as the lamp is broken down because there is a high enough low frequency energy to push the lamp from glow to arc. The new igniting control scheme is also utilized for CCM to reduce the circulating current at the starting mode. In the circuit, the output L, C, lamp resistor R network is a variable band-pass filter. When the filter is operating as a high Q circuit, the filter is a high frequency band-pass filter which allows resonant frequency pass through; when the filter is operating as a low Q circuit, the filter is a low pass filter. For the network, if we set up the exciting source with high frequency and low frequency components, the lamp resistor can be used to control the variable band-pass filter. That is, when the lamp resistance is high at the start-up mode, the network is a high Q high frequency band-pass filter; when the lamp resistance is low at steady-state mode, the network is a low Q low pass filter. In this way, the characteristics of the filter can automatically be controlled. Thus, an exciting source with high frequency and low frequency components is produced by the present new igniting control scheme. This exciting source can drive the network to produce a high enough high frequency voltage to break down the lamp and to transfer the low frequency energy to push the lamp from the glow to the arc automatically.
In the circuit, the output inductor has two functions. One is to work as a resonant inductor during lamp starting; the other is to work as a buck inductor to regulate the lamp power and the lamp current during lamp normal operation. Generally speaking, these two functions are contradictory. As a resonant inductor, it is required that the inductor value be low to obtain a low output impedance of the network; and as a buck inductor, it is required that the inductor value be high to make the output current ripple low to avoid the-acoustic resonance. There are some known ways to solve this problem. The easy way is changing the switching frequency between ignition mode and the steady state mode. That is, in the ignition mode, the switching frequency is low to obtain the low output impedance of the network; and in the steady state mode, the switching frequency is high to make the output current ripple low to avoid acoustic resonance. The cost of this method is an increase in switching loss and a decrease in the whole system efficiency. With the new igniting control scheme, the value of the inductor used for resonant ignition can be high and the switching frequency only increases a little bit in the steady state mode, and the inductor can still make the output current ripple low to avoid the acoustic resonance. It is good to keep high efficiency for the whole system.
Other advantages and features will become apparent from the following description, and from the claims.